How do you explain TTP to patients?

I tell them there is an enzyme in their blood called ADAMTS13 that helps keep their platelets from sticking together and plugging small blood vessels, and that for unknown reasons, their own immune system is destroying this enzyme. The symptoms of TTP are caused by clumps of stuck-together platelets keeping blood from getting to the brain and other organs, and by consumption of platelets during this process leading to a very low platelet count that causes bruising and bleeding.

I then go on to explain the rationale for plasma exchange (replace missing ADAMTS13, remove the antibodies that destroy it) and immunosuppression (stop the immune system from destroying ADAMTS13).

You can always dress this explanation up a bit if the patient is comf...